Process for producing phosphors



United States PatentO PROCESS FOR PRODUCING rnosrnons Paul Goldberg, Long Beach, and Allen L. Solomon, Glen Cove, N.Y., assignors, by mesne assignments, to Sylvania Electric Products, Inc., Wilmington, Del.,- a corporation of Delaware No Drawing. Filed Dec. 30, 1957, Ser. No. 705,750

12 Claims. (Cl. Za -301.6)-

Our, invention relatesto electroluminescent phosphors and processes for makingthe same.

Electroluminescent phosphors are phosphors which emit light in the presence of an electric field. One type of phosphor, known as the zinc sulphide type of electroluminescent phosphor, includes together with suitable activators and coactivators, those phosphors wherein up to-2O mole percent of: cadmium can be substituted for an mole percent of selenium can'be substituted for an equivalent' mole percent of sulphur. Conventiona1ly, such phosphors are dispersed in dielectric media and the dispersion is placed between two electrically conductive electrodes, at least one of which permits the passageoflight thereth'rough to producean electroluminescent lamp. A voltageis applied between the two electrodes and light is emitted from-the dispersion. 7

'We" have succeeded indevelopinga new process for producing electroluminescent phosphors of the zinc sulphide type which results in a phosphorwithsignificantly enhanced electroluminescence.

A'ccordingly, it'is an object of the present invention to, provide a-new and improved process for making electrolur'ninescent phosphors of the zinc sulphide type.

Ahotlier objectis, to increasethe brightness of an electroluminescent phosphor'of the zinc sulphide type.

Still another object is to improve the processing of electroluminescentphosphors of'the zinc sulphide type in y manner as'to sharply increase the brightness of the resultant-phosphor. V

I 'h'ese and otherobjects of our invention will either be explained or will-become apparenthereinafter.

'In'accorda'nce with the principles of our invention, a I copper activator and at least, one halide coactivator & selected from the class'consisting'of'chloride, iodide'and bromide are added to a basematerial of the zinc sulfide type wherein upto 20 mole percent of" cadmium can be wherein u'pjto 30 mole percent of selenium? can be sub stitut'ed' for an' equivalent mole percent of sulfur.. 'I he resultant mixture is then fired to a temperature falling approximate range 1000-1300" C, to produce a ermediate phosphor product having a copper activae to concentration falling within the approximate range 1 to 5x10 gram atoms permole of base mateanda halide coactivator concentration also falling the'approximate range 1x10- vto 5 X107 gram er mole of-base material. I ,l intermediate product is ,then' mixed with at least gram atoms per mole of copper and refired i-n inaninert atmosphere at av lower temperature gfwithin the approximate range 750 C.-95 0,' C. to produce after suitable washing, the. final, phosphor prod? uct. Y-This'final product, when testedin an electrolumines lamp, has significantly increased electroluminescence, ght-output being; increased bya factor; of two or subiftituted for an, equivalent mole percent of zinc, and.

2,982,740. Patented May 2, 1961 equivalent mole percent of zinc and'wherein up to"3'0% In our process, the activator-coactivatormole ratio of the intermediate product resulting from the first firing step isdetermined prior to the second firing step; If this. mole ratio for the intermediate product is substantially lessthan 1, an appropriate amount of copper must be added to readjust the mole ratio to approximately one in the second firing step. When this mole ratio is substantially equal to or larger than 1, at least the minimum amountv of copper previously stated, i.e. 1.0 l0- gram atoms per mole, must be added in the second firing step. If copper is added in an amount larger than this mini mum, it is preferable to remove the excess copper fromthephosphor produced as a result of the second firing step, as for example by washing the phosphor with an aqueous solution containing about 0.1% by weight of cyanide. (If this excess copper is not removed, there can be substantial reductionof emitted light due to light absorption in the phosphor itself.) If copper is not added in both firing steps, we have found that the desired enhancement of electroluminescence will-not ensue.

Illustrativeembodiments of our invention will now be described in more detail with reference to the examples which follow.

Example I Zinc.-sul fide and cadmium sulfide powders were thoroughly mixed together, the mixture containing 95% by weight vof zinc sulphideand 5% by weight of cadmium sulfide and weighing 15 grams. Copper sulfate was thoroughly blended with this mixture in an amount suflicient to establish a copper concentration of 2 10* gram sisting of 3% barium chloride, 3% magnesium chloride and-2 sodium chloride, each percentage being by weightof the sulfide mixture. The resulting blend was fired in a coveredcrucible in air to a temperature of 1150 C.

forsix hours.

The intermediate product resulting from this firing was irradiated with ultraviolet and found to exhibit green phosphorescence. When chemically analyzed, this product was found to have an activator-coactivator,mole ratio 0f-.,8; with no loss of the added copper.

Copper sulfate was added to the intermediate product in an amount sufiicient to increase the total copper concentration by 1X10" gram atoms per mole of zinc sulfide. The copper-augmented mixture was then fired in acovered crucible in air to a temperature of 800 C. for aboutone hour. The furnace was shut off, and the crucible was permitted to-cool for several hours inthe furnace before removal; The mixture was then removed from the crucible and washed with dilute'acetic acid to produce the final product, an electroluminescent phose phor having an activator-coactivator mole ratio of 1.2; This phosphor, when tested in an electroluminescent lamp, was found to have enhanced electroluminescence as compared to known electroluminescent phosphor havingsimilar color characteristics, when all phosphors were tested under the same conditions in electroluminescent i we as comp dhe s tp p t p 'a g le her r1195:

lamps. More, specifically, the light output of our phosphor when dispersed in castor oil and placed in a lamp 1: inch wide by 1 inchlong by 0,005 inch thick and actuated by-an alternating voltage of 605600 volts,(the frequency being"60-- 6000 cycles per second) was about T twotimes larger than the, light output from the best of these known phosphorswhen measured byfthefsamc photometer underthesame conditions. v.

Our-phosphor was found to havea much lighterbody color than the: aforementioned prior art phosphors and exhibited a higher degree of crystallinity. X-ray powder Example 11 '1 The process of Example I was repeated using the same amounts of activator and coactivator, the base material again weighing grams and containing 85.5% by weight of zincsulfide and 14.5% by weight of cadmium sulfide. The results were essentially the same as in Example I, exceptthat this phosphor exhibited blue-green electroluminescence.

Example Ill 7 Copper sulfate was thoroughly blended with zinc sulfide in an amount sutficient to'establish a copper concentration of 9X10" gram atoms per mole of-the zinc sulfide. In addition, the chloride flux of Example I was also thoroughly blendedwith the mixture, in an amount equal to about 8% of the sulfide mixture. The resulting blend was fired in a covered crucible in air to a temperature of 1200 C. for three hours. i a

' The intermediate product resulting from this firing was irradiated withultraviolet and found-to exhibit "green fluorescence. When chemically analyzed, this product was found to have an activator'coactivator mole ratio of 1.1 with no loss of the added'copper. w i

Copper sulfate was added to the intermediate product in an amount sufiicient to increase the total copper concentration by 6 10- gram atoms per. mole of zinc sulfide. The copper-augmented mixture was then fired in a covered crucible inair to a temperature of about 775 C. for about one hour. After the crucible was permitted to cool in the furnace in the manner previously indicated, the mixture was then removed from. the crucible and washed first with dilute acetic acid, then washed thoroughly with Water, then washed with an aqueous potassium cyanide solution (0.1% concentration by weight) and finally again washed with water to produce thefinal product, an electroluminescent phosphor having an activator-coactivator mole ratio of 1.0. This phosphorv when tested in an electroluminescent lamp, was found to have enhanced electroluminescence colored green at low excitation frequencies (up to about 1.5 kilocycles/sec.), the color gradually changing toward blue at higher frequencies.

' Example IV Zinc sulfide, selenium and zinc oxide powders were mixed together, the mixture containing 66.1% by weight of zinc sulfide, 20.1% of selenium, and 13.8% of zinc oxide. Copper sulfate was blended with this mixture in an amount sufiicient to establish a copper concentration .4 color gradually changing to green as the excitation frequencies were increased.

Example V The process of Example IV was repeated with the copper addition in .the first firing step being about 7 10- and with about 13% by weight of potassium bromide being substituted for the 17% by weight of potassium iodide.

tion by 1X10- gram atoms per mole in place of the 7 l0-- gram atoms per mole addition of Example IV.

The results were substantially the same as in Example IV, except that the phosphor produced exhibited greenish-yellow electroluminescence.

Additional tests indicated that any mole percent (up to 30 mole percent) of selenium can be substituted for an equivalent mole percent of sulfur, and any mole percent up to 20 mole percent of cadmium can be substituted for an equivalent mole percent of zinc.

Further tests on our process showed that the firing temperatures used on the first step could be varied within the approximate range 1000 0-1300 C. with an optimum of about 1150 C.1200 C.; that the firing temperatures in the second firing step could be varied Within the approximate range 700 C.-950 C. with an optimum range of 'about 800 C.-850 C.; that the absolute con centrations of the copper and the halide in the intermediate product could be varied .within the approximate range 1X 10- to 5+10' gram atoms per mole of the base material. a V a xWhile we have shown and pointed out our invention as applied above, it will be apparent to those skilled in the art that many modifications can'be made within the scope and sphere of our invention as defined in the claims which follow.

What is claimed is:

1. A process for producing an electroluminescent phos-.

phor from a base material of the zinc sulfide type wherein up to 20 mole percent of cadmium can be substituted for an equivalent mole percent of zinc and wherein up to 30 mole percent of selenium can be substituted for an equivalent mole percent of sulfur, said process comprising the steps of mixing said material with a copper activator and at least one halide coactivator selected from the group consisting of chloride, iodide and bromide; firing said mixture in a non-reducing atmosphere to a temperature falling within the approximate range 1000 of about 3.2 10- gram atoms per mole of the mixture.

In addition, potassium iodide wasalso blendedwith the irlixture in an amount equal-to about 17% by weight of the mixture. The resulting blend was fired in a covered crucible in helium (or other inert atmosphere) to a tem- 'perature of 1165 C. for about one hour. 1'

After washing with acetic acid, theinterrnediate product was found to contain 30 mole percent of zinc selenide and 70 mole percent of zinc sulfide.

' Copper sulfate was added to the intermediate product p in' an amount sufficient to; increase the total copper concentration by 7 10- gram atoms per mole of the mixtur e. The copper-augmented mixture was; then fired in helium to a temperature of 800 C. for: about 40 minutes. The v.r'nix-ture was then removed fromthe crucibleand washed first with an aqueous solution of potassium cyanide (0. 1% concentration by weight) and then washed with water 'to produce the final product, an'electroe C.-l300 to produce an intermediate product having absolute activator and coactivator concentrations falling within the same approximaterange of 1X10- to 5 X 10'. gram atoms per mols of said material; mixing at least 1X 10- additional gram atoms per mole of said activator with said intermediate product; and firing said copperaugmentecl intermediate product in a non-reducingatmosphere to a temperature falling within the approximate range 700 C.950"v C. to produce said electroluminescent phosphor. I e a 2'.'.The process for manufacture of an electroluminescent zinc sulfide type phosphorhaving a predominantly hexagonal crystal structure, which process comprises the steps of thoroughly mixing precipitated-zinc sulfide and i cadmium sulfide in proportions togive from 0 to 20 mole percent cadmium sulfide in the mixture with copper 'sulfate in an amount such that 1 10: up to. 5 X 10- gram atoms ofcopper are present for each mole of the mixture of zinc and cadmium sulfide and from 3-15% of at least one fluxing material selected from the group consisting luminescent phosphor. This phosphor, when tested in an electroluminescent"lamp, was found to have enhanced yellow-green electroluminescence at: excitation frequen- 'cies of about 1000 cycles" per second and lower;the

of bromides, chlorides and iodides of sodium, barium, potassium and magnesium, firings'aid mixture in covered crucibles in air at a temperature between about 1000 C and 1300 C.; cooling the fired mixture rapidly, by re moving thecruciblefrom the furnace; washing the cake, with'watery adding between 1 X l0- to '5 x10 gram atoms ofcopper per mole of zinc sulfide -tosaid cake; I

Further, the copper addition in the, second firing step increased the total copper concentra- 5. refiring'saidc'ake ina covered crucible in air, ata temperature bctweenabout 700 C. and.950C;;vwashing* the resultant 'cakefirst witha' "dilute aqueous solution" of aceticacid; then withiwater; then with a dilute solution of an alkali cyanide, and finallyagain washing with water to remove excess cyanide along with dissolved copper; and drying the resultant phosphor;

3. A-process for producingan electroluminescent phos= phor'from a-bas'e, material of the zinc sulfide. type where* in up'to' ZO fmOlepe'rcent' of cadinium' can be: substituted for-an equivalent 'molejpercenhofz'inc and wherein up to'@ 30 mole" percent of selenium can be substituted for equivalent'mole percent of sulfur', said proces's co'mpris ing-the steps of mixing saidmaterialwith'a'copper acti vator and at f least one vhalide coac'tiva tor selected from the group co'nsistingfof chloride; iodide andbromide; fir--- ing 'said1mixture in anon-reducing at'mo'sp-h'eretoa tem within the approximate range 700 C.950 C. to produce said electroluminescent phosphor; and chemically removing sufficient copper from said phosphor to readjust said ratio to about 1.

4. A process for producing an electroluminescent phosphor from a base material of the zinc sulfide type wherein up to 20 mole percent of cadmium can be substituted for an equivalent mole percent of zinc and wherein up to 30 mole percent of selenium can be substituted for an equivalent mole percent of sulfur, said process comprising the steps of mixing said material with a copper activator and at least one halide coactivator selected from the group consisting ofchloride, iodide and bromide; firing said mixture in a non-reducing atmosphere to a temperature falling within the approximate range 1000 C.- 1300 C. to produce an intermediate product having absolute activator and coactivator concentrations respectively falling within the same approximate range of 1X10- to X10" gram atoms per mole of said base material, the activator-coactivator mole ratio falling below 1.0; mixing at least 1X10" additional gram atoms per mole of said activator with said intermediate product to adjust said ratio to about 1.0; and'firing said copperaugmented intermediate product in a non-reducing atmosphere to a temperature falling within the approximate range 700 C.950 C. to produce said electroluminescent phosphor.

5. A process for producing an electroluminescent phosphor from a base material of the zinc sulfide type wherein up to 20 mole percent of cadmium can be substituted for an equivalent mole percent of zinc and wherein up to 30 mole percent of selenium can be substituted for an equivalent mole percent of sulfur, said process comprising the steps of mixing said material with a copper activator and at least one halidecoactivator selected from Q the group consisting of chloride, iodide and bromide; firing said mixture in a non-reducing atmosphere to a temperature falling within the approximate'range 1000 C.- 1300? C. to produce an intermediate product having absolute activator and coactivator concentrations respectively falling within the same approximate range or 1x10" to 5 l0 lgram atoms per mole of said base material, the activator-coactivator mole ratio being in excess of 1.0; mixing atleast 1 10- additional gram atoms per mole. of said activator with said intermediate product;

firing said copper-augmented intermediate product in a electroluminescent phosphor; and washing said phosphor in a cyanide solution to remove suflicient copper from;

saidphosphorto readjustsaid ratio toabout 1L0."

6. A- process for producing; an" electrolu minescent- P phosphor from; at least" one-basematerial selected-horn the-class{ consistingof the'- sulfides of zinc and cadmium-- zinc'wherein up'to mole percent of cadmium canbe} substituted for an equivalent mole percent ofzinc, said process comprisingthe'steps fof mixing a copper activator and at-leastj -one-lialide"coactivator selected from the group consisting of chloride; iodide and bromide; firingsaid'- mixture in air toiia temperature falling withinthe ap-= proximate range 1000 (II-1300" C. to produce an in- ,termediate product 'liaving absolute activator andco activator concentrations respectively: falling within the same approximate range of ',l 10" to- 5 1Q-' gram" atoms per nioleof said basefmaterial; mixing at le'ast' 1x107 additional gram-- atoms per moleof said acti vat 'or' with sa id intermediate product; and firing said'-'- copper-augmented intermediate product hair to a-tem perature fallingwithin the" approximate' range 700 j C.-- 950 CI to p rc iduine. said"el'ectr jlurninescentphosphor.-

'7.A"" 'proces'sfor producing" an electroluminescent phosphor-from at'f least one) base materiahselected frontthe group consisting-of the' sul fo'selenides of zinc and zinc-cadmium wherein; u 20 -moleper'cent of cadmium? can 'be"substituted for "anrequivalent-mole percent-of zinc and wherein upto 30 mole percent of selenium can be substituted for an equivalent mole percent of sulfur, said process comprising the steps of mixing a copper activator and at least one halide coactivator selected from the group consisting of chloride, iodide and bromide; firing said mixture in an inert atmosphere to a temperature falling within the approximate range 1000 C.1300 C. to produce an intermediate product having absolute activator and coactivator concentrations respectively falling within the same approximate range of 1X10- to 5 10- gram atoms per mole of said base material mixing at least 1 10- additional gram atoms per mole of said activator with said intermediate product; and firing said copper-augmented intermediate product in an inert atmosphere-to a temperature falling within the approximate range 700 C.-950 C. to produce said electroluminescent phosphor 8. A process for producing an electroluminescent phosphor from a base material mixture composed of about 95% by weight of zinc sulfide and about 5% by weight of cadmium sulfide, said process comprising the steps of adding a copper activator in an amount equal to about 2X10- gram atoms per mole of mixture and adding about 8% by weight of mixture of a chloride c0- activator composed of the chlorides of barium, magnesium and sodium; firing the resultant mixture in air to a temperature of about '1150 C.; mixing about 1 10 additional gram atoms per mole of mixture of said activator with said intermediate product; firing said copper-augmented intermediate product in air to a temperature of about 800 C, to produce a blue electroluminescent phosphor.

9. A process for producing an electroluminescent phosphor from a base material mixture composed of about 85 by weight of zinc sulifide and about 15% by adding about 8% by weight of mixture of a chloride coactivator composed of the chlorides of barium, magnesuium and sodium; firing the resultant mixturein air I to a temperature of about 1150 C.; mixing about non-reducing atmosphere to a temperature 'falling within 1 10 additional gram atoms per mole of mixture of said activator with said intermediate product; firing said. copper-augmented intermediate product in air to a temperature of about 800 C; to produce a blue-green electroluminescent phosphor.

10 A process for producing an electroluminescent 7 I m phosphor from zinc sulfide base material, said process comprising the steps of addinga copper activator in an amount equal to about 9 10-- gram atoms per'mole of said material and adding about-8%-by weightof said materialof a chloride coactivator composed of the chlorides of barium, magnesium and sodium; firing the resultant mixture in air to a temperature of about 1200 C.; mixing about 6x10- addition a1 gram atoms per mole of mixture of said activator with said intermediate product; firing said copper-augmented intermediate product in air to a temperature of about 775 C. to produce a green electroluminescent phosphor.

-11. A process for producing an electroluminescent phosphor from a base material mixture composed of about 66% by weight of zinc sulfide, aboutp20% by weight of selenium and about 14% by weight of zinc oxide, said process'comprising the steps of adding a copper activator to said mixture in an amount equal to about 3.2 10-' gram atoms, per mole of said mixture and also adding potassium iodidein: an amount equal to about 17% by weight ofsaid mixture; firing the oresultant mixture in aninert; atmosphereto a temperature of about 1165" 0.; adding vsufiicientcopper activator to the intermediate product to increase the copper content by about -7 10- gram atoms per mole .of said product; and firing said copper-augmented intermediate product in an inert atmosphere to a temperature of phosphor from a base material mixturecomposed of about 66% by-weight of zinc sulfide, about 20% by weight of 'selenium and about 14% by weightof zinc oxide, said process comprising the steps of adding a copper activator to saidmixture in an amount equal to about 7X10- gram atoms per mole of said mixture and, also adding potassium bromide in an amount equal to about 13% by weight of said mixture; firing the resultant mixture in an inert atmosphere to atemperature of about 1165 C.; adding suflicient copperactivator to the intermediate product to increase the copper content by about 1x10" gram atoms per mole of said product; and firing said copper-augmented intermediate product in an inert atmosphere toatemperature of about 800 C. to produce a greenish-yellow: electroluminescent phosphor.

References Cited in the file of this patentv A UNITED STATES PATENTS 7 2,774,739 Butler Dec.18,'1 956 Butler Aug. 13, 1957 2,821,509 Hunt Ian. 28, 1958 

2. THE PROCESS FOR MANUFACTURE OF AN ELECTROLUMINESCENT ZINC SULFIDE TYPE PHOSPHOR HAVING A PERDOMINANTLY HEXAGONAL CRYSTAL STRUCTURE, WHICH PROCESS COMPRISES THE STEPS OF THOROUGHLY MIXING PRECIPITATED ZINC SULFIDE AND CADMIUM SULFIDE IN PROPORTIONS TO GIVE FROM 0 TO 20 MOLE PERCENT CADMIUM SULFIDE IN THE MIXTURE WITH COPPER SULFATE IN AN AMOUNT SUCH THAT 1X10-4 UP TO 5X10-3 GRAM ATOMS OF COPPER ARE PRESENT FOR EACH MOLE OF THE MIXTURE OF ZINC AND CADMIUM SULFIDE AND FROM 3-15% OF AT LEAST ONE FLUX MATERIAL SELECTED FROM THE GROUP CONSISTING OF BROMIDES, CHLORIDES AND IODIDES OF SODIUM, BARIUM, POTASSIUM AND MAGNESIUM, FIRING SAID MIXTURE IN COVERED CRUCIBLES IN AIR AT A TEMPERATURE BETWEEN ABOUT 1000*C. AND 1300* C., COOLING THE FIRED MIXTURE RAPIDLY BY REMOVING THE CRUCIBLE FROM THE FURNACE, WASHING THE CAKE WITH WATER, ADDING BETWEEN 1X10-5 TO 5X10-3 GRAM ATOMS OF COPPER PER MOLE OF ZINC SULFIDE TO SAID CAKE, REFIRING SAID CAKE IN A COVERED CRUCIBLE IN AIR AT A TEMPERATURE BETWEEN ABOUT 700*C. AND 950*C., WASHING THE RESULTANT CAKE FIRST WITH A DILUTE AQUEOUS SOLUTION OF ACETIC ACID, THEN WITH WATER, THEN WITH A DILUTE SOLUTION OF AN ALKALI CYANIDE, AND FINALLY AGAIN WASHING WITH TO REMOVE EXCESS CYANIDE ALONG WITH DISSOLVED COPPER AND DRYING THE RESULTANT PHOSPHOR. 